1. Field of the Invention
The present invention relates to a noise filter for effectively removing noise flowing in a signal wire disposed on a circuit board, and a noise filter array.
2. Description of the Related Art
Depending on the communication system of a portable telephone, for example, a single portable telephone may use a plurality of communication bands. In order to prevent degradation of reception sensitivity in each of the communication bands, it is necessary to effectively remove noise in each frequency band.
Known examples of a noise filter used for such noise removal include a choke coil, a ferrite bead, and a ladder-type LC filter.
When the choke coil mentioned above is used as the noise filter, noise countermeasures can be easily implemented because noise can be removed by simply connecting the choke coil to each signal wire. However, the choke coil can only remove noise at a specific frequency because the band for which noise removal can be performed is relatively narrow, which disadvantageously makes it difficult to remove noises in a plurality of frequency bands at the same time.
Further, when a ferrite bead is used, as in the case of the choke coil, noise countermeasures can be easily implemented because noise can be removed by simply connecting the ferrite bead to each signal wire. However, since a ferrite bead removes noise even in a low frequency band, it exerts a large influence on the signal waveform such as by attenuating a necessary signal. Further, since high attenuation cannot be attained, it may be impossible to achieve a satisfactory noise removal effect.
Further, the ladder type LC filter mentioned above comes in various types, such as a T type, a π type, or an L type. Although any one of the above-described types of ladder type LC filter can provide wide-band noise removal characteristics through appropriate setting of the inductance and capacitance, since it is necessary to ground an external electrode connected to a capacitor, it is essential to form a grounding electrode pattern on a circuit board to which the ladder type LC filter is mounted. This disadvantageously limits the freedom of wiring layout on the circuit board.
Further, while a plurality of signal wires are formed on a circuit board involving high-density mounting, depending on the component layout, it may be difficult to form grounding electrode patterns having a sufficient line width together with these signal wires. As a result, due to the influence of a parasitic inductance in the grounding electrode patterns, the frequency characteristics of the ladder type LC filter change, which disadvantageously makes it impossible to remove noise in a satisfactory manner.
On the other hand, in the related art, there has been proposed a noise filter constructed as follows (see, for example, Japanese Unexamined Patent Application Publication No. 5-267059). That is, the noise filter includes a filter element composed of one trap circuit formed by the inductance of a coil, which is composed of a plurality of coil conductors laminated in a spiral fashion within a dielectric, and a floating capacitance between the coil conductors. On either side of this element, a filter element composed of one trap circuit formed by the inductance of a coil, which is composed of a plurality of coil conductors laminated in a spiral fashion within a magnetic material, and a floating capacitance between the coil conductors, is arranged, and these filter elements are integrated with each other to thereby form the noise filter.
According to this noise filter, the resonance frequency of the trap circuit constituting each of the filter elements is set to correspond with each of a plurality of communication bands, thereby making it possible to remove noise in each of the communication bands.
However, in the noise filter described in Japanese Unexamined Patent Application Publication No. 5-267059, since not only the resonance frequency on the high frequency side but also that on the low frequency side is dependent on the floating capacitance generated between the coil conductors, it is not always easy to perform noise removal in an appropriate and satisfactory manner for each frequency band.
That is, in an LC parallel resonant circuit, the resonance frequency is dependent on the value of the LC product; the larger the LC product, the smaller the resonance frequency. Here, the setting of the resonance frequency on the high frequency side can be readily realized by adjusting the floating capacitance because the LC product may be set to be small. On the other hand, for the setting of the resonance frequency on the low frequency side, the LC product must be set to be relatively large. In this case, since problems such as distortion of the signal waveform occur when the value of the inductance L is set too large, there is naturally a limit as to how large the value of the inductance L can be set. Therefore, in order to compensate for the shortage of the inductance L, it is necessary to obtain a relatively large floating capacitance by reducing the inter-layer distance between the coil conductors or by changing the insulation material.
However, when the inter-layer distance between the coil conductors is reduced as described above, this causes deterioration in characteristics or reliability. Further, in the case where the insulation material is changed, there is a problem in that the number of manufacturing man-hours increases due to the occurrence of delamination depending on the characteristics of the material or due to an increase in the kinds of sheets to be used. In the case of Japanese Unexamined Patent Application Publication No. 5-267059, in particular, because it is necessary to fire the dielectric and the magnetic material at the same time for integration, there is a problem in that not only is the reliability in terms of strength low due to cracks, peels, or the like that are liable to occur during the manufacturing process, but also an increase in cost is caused due to the necessity of setting and managing the optimum manufacturing conditions with high precision.
Further, in the related art, there has been also proposed a construction in which a plurality of coils each composed of a plurality of coil conductors laminated in a spiral fashion within a single dielectric are formed at the same time to thereby form a plurality of trap circuits.
However, as in the case of Japanese Unexamined Patent Application Publication No. 5-267059 described above, also in the case of the noise filter of this construction, it is difficult to set each trap circuit to a desired frequency in correspondence with a plurality of communication bands, and further, there maybe cases where the coils tend to readily magnetically couple with each other. Thus, a plurality of trap circuits cannot be formed or high attenuation cannot be attained at the resonance frequency of each trap circuit, which disadvantageously makes it impossible to perform noise removal in an appropriate and satisfactory manner for each frequency band.